Stainless steel product having excellent antibacterial activity and method for production thereof

ABSTRACT

Stainless steel having superior antibacterial and corrosion resistance properties is provided by adding and dispersing optimal amounts of titanium and copper in the stainless steel. Corrosion resistance analysis and antibacterial analysis were performed to prove the corrosion resistance and antibacterial properties thereof. According to the present invention, methods to make the stainless steel having superior antibacterial and corrosion resistance properties are also provided. The methods use a base stainless steel such as austenitic, ferritic or martensitic stainless steel. The steel can be hot rolled, cold rolled, annealed, pickled, and heat treated.

FIELD OF THE INVENTION

The present invention relates to a stainless steel with good antibacterial property in conjunction with good corrosion resistance and a manufacturing method thereof, and especially to antibacterial stainless steel suitable for use in, for example, kitchen fixtures, medical apparatus, electrical appliances, chemical apparatus, and building materials.

BACKGROUND OF THE INVENTION

Stainless steel is used widely in cooking facilities for food safety. However, after cleaning, the surface of stainless steel still retains considerable quantities of microorganism that could be a source of diseases. Many technologies have been used to deal with these problems.

For example, a method utilizing antibacterial reagent coated on the surface of stainless steel to restrict the growth of microorganism is provided in U.S. Pat. No. 5,997,815, which is critical to minimize the spread of microbes that cause infection. However, the coating on the surface is stripped and removed by scrubbing or abrasion during use or surface polishing and the antibacterial properties thereof are gradually reduced and do not last over long periods of time. In this method, additional process steps are required to make the coating layer.

In order to solve the problems described above, various stainless steel composition have been proposed, including austenitic stainless steel with 2% or less by volume copper added, as disclosed in Japanese Patent No. JP2000303152; martensite stainless steel with 2.7 to 5% by weight copper added, as disclosed in Japanese Patent No. JP11092884; and ferritic stainless steel with 0.1 to 5.0% by weight copper added, as disclosed in Japanese Patent No. JP10324920. However, in the technologies disclosed in Japanese Patent Nos. JP2000303152, JP11092884, and JP10324920, the antibacterial properties resulted from the copper ions leaching from the surfaces of the steel sheets as shown in FIG. 1. The leaching of copper in ionic form is also due to breakage of passivation layers at the leaching points. Furthermore, the precipitation of Cr₂₃C₆ in grain boundaries shown in FIG. 2 is due to the greater affinity between carbon and chromium, and prevents the passivation layers from re-synthesizing. Therefore resistance to corrosion is seriously degraded, even though antibacterial properties are improved. Accordingly, it is difficult for stainless steel having copper therein to have antibacterial properties and corrosion resistance at the same time.

A stainless steel having excellent antibacterial activity without adding copper is provided in U.S. Pat. No. 6,306,341 and U.S. Pat. No. 6,391,253. However, additional amounts of precious metals such as silver, silver oxide, platinum and vanadium must be added. The cost of adding silver, platinum or vanadium is greater than the addition of copper.

SUMMARY OF THE INVENTION

Objects of the present invention are to provide stainless steel and manufacturing methods therefore that resolve the problems in the conventional technologies, without additional process steps, with a competitive cost, and improving its antibacterial properties.

Stainless steel with superior antibacterial and corrosion resistance properties is provided by adding and dispersing optimum amounts of titanium and copper in the stainless steel according to present invention. Evidence of corrosion resistance and antibacterial properties is provided by corrosion resistance analysis performed by analyzing the microstructure of stainless steel sheet with scanning electron microscopy(SEM), transmission electron microscopy (TEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD), and antibacterial analysis.

The methods for forming a stainless steel with good antibacterial properties and corrosion resistance starting from a base stainless steel is also provided by the present invention. The base stainless steel is austenitic, ferritic or martensitic stainless steel. The steps of process include hot rolling, cold rolling, annealing, pickling, and heat treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 illustrates the copper ions leaching from the surfaces of the steel sheets;

FIG. 2 illustrates the formation of Cr₂₃C₆ in the grain boundaries;

FIG. 3 illustrates the lattice pattern of TiC in the alloy through TEM electron diffraction analysis;

FIG. 4 illustrates the SEM microstructure of the stainless steel after etching referring to blank test;

FIG. 5 illustrates the chromium distribution after SEM mapping versus FIG. 4 according to one embodiment of the present invention;

FIG. 6 illustrates the optical microstructure of a steel surface by optical microscopy after the electrolytic etching referring to blank test;

FIG. 7 illustrates a optical microstructure of a steel surface by optical microscopy after the electrolytic etching according to one embodiment of present invention; and

FIG. 8 illustrates the histogram of the antibacterial analysis in number of living bacteria.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

According to an embodiment of the present invention, copper and titanium are added to the formulated base steel powder in, for example, a furnace. Accordingly, the amounts of chromium, titanium, and of copper are respectively 18-30% by weight, 0.3-1% by weight, and 0.3-3.0% by weight in molten stainless steel.

The base steel of the present invention is austenitic, ferritic or martensitic stainless steel. In an embodiment of the present invention the chemical composition of the ferritic stainless steel is preferably as follows: 0.03-0.05% by weight carbon, less than 0.75% by weight silicon, less than 1.0% by weight manganese, less than 0.04% by weight phosphorus, less than 0.030% by weight sulfur, 15-20% by weight chromium, less than 0.08% by weight nitrogen, and the balance being iron and incidental impurities. In addition, one or more elements listed as follows can be added to the ferritic stainless steel: aluminum, less than 0.01% by weight; nickel, less than 0.5% by weight; molybdenum, less than 0.05% by weight; titanium, less than 1.0% by weight; niobium, less than 1.0% by weight; zirconium, less than 1.0% by weight; copper, less than 3.0% by weight; tungsten, less than 0.30% by weight; and boron, less than 0.01% by weight.

The present invention was accomplished based on further research hereunder. The passivation layer is formed of chromium oxide, for example Cr₂O₃ on the surface of the stainless steel. The leaching of copper in ionic form causes breakage of the passivation layers. The passivation layer can be repaired by the re-synthesis of Cr₂O₃ to maintain the corrosion resistance of the stainless steel. The chromium content is not less than 12% by weight. The chromium content is determined as the passivation layer of corrosion resistance. The corrosion resistance is low when the chromium is less than 12% by weight. The upper limit of chromium preferably is less than 50% by weight; however, more than 30% causes an ugly appearance and poor workability. In a preferable embodiment the chromium content is 18% by weight.

Copper content is 0.3-3% by weight. The copper is the most important element of the present invention, having an inhibitory effect on bacterial growth and enhancing antibacterial properties. These effects of the copper are observed at amounts of not less than 0.3% by weight; however, when the copper content exceeds 2% by weight, corrosion resistance is degraded. In a preferable embodiment, a stainless steel, having antibacterial properties, comprises 3% copper by weight.

The titanium content is 0.1-1% by weight. The affinity between titanium and carbon is greater than the affinity between chromium and carbon. In a preferable embodiment, the titanium appears as TiC. FIG. 3 illustrates the pattern of the lattice on the surface of the alloy as detected by convergent beam electron diffraction analysis. With reference to JCPDS-INTERNATIONAL CENTRE FOR DIFFRACTION DATA, the lattice pattern of precipitate shown in FIG. 3 has been proven as TiC. The precipitation of TiC may prevent the precipitation of Cr₂₃C₆, which could create a chromium depleted zone in grain boundaries. FIG. 4 and FIG. 5 illustrate the microstructure of the stainless steel, detected by, for example, scanning electron microscopy (SEM). According to a comparison with the blank test (shown in FIG. 4), Cr₂₃C₆ is precipitated as shown in the blank test, which has no titanium added. According to energy dispersive spectrometry (EDS) analysis, the leaching of copper increases corresponding to the increased titanium content on the surface of the stainless steel. In an embodiment of the present invention, the content of titanium in the stainless steel may be up to 3% by weight without degrading the corrosion resistance.

A conventional hot rolling process at 1050° C. or less may be used. In a embodiment, before hot rolling the temperature of the cast steel may be maintained at 1050° C. for at least 50 minutes. The cast steel may be rolled with a reduction of 50%-60%. The hot rolled steel strip is subjected to annealing, which maintains the temperature for at least 8 hours before cooling to room temperature. An pickling agent, preferably in a 55 wt %, may be used to pickle the annealed steel prior to a cold rolling. The pickling agent comprises, for example, HNO₃ and HF, and the ratio of HNO₃ to HF is preferably 1:4. The cold rolling may be carried out on the pickled steel strip with a reduction of 50%-60%. After a heat treatment at 600-800° C. lasting for at least 30-120 minutes, the stainless steel having antibacterial and corrosion resistance properties is complete.

Corrosion resistance analysis is performed by immersing the stainless steel in a 10 wt % oxalic acid solution for electrolytic etching. The microstructure of the stainless steel may be mapped with scanning electron microscopy (SEM) and the corrosion pattern may be analyzed with optical microscopy. FIG. 7 illustrate the optical microstructure according to optical microscopy after the electrolytic etching of a steel surface with 0.6% titanium by weight added. Comparing to FIG. 6 as a blank test, there appears some grain boundaries. Formation of chromium carbide is thus avoided by adding optimal amounts of titanium. Further, the problem of providing a stainless steel having copper therein and having corrosion resistance at the same time is resolved.

Antibacterial analysis composes the steps hereunder:

A test piece having an area of 20 cm² is washed and degreased with absorbent cotton containing 99.5% ethanol. Staphylococcus aurous is dispersed in a 1/50 nutrient broth (NB) solution and cultivated for 24 hours. The NB solution is then diluted with phosphate buffer solution (PBS) to insure that the PBS has a concentration of Staphylococcus aurous of 10⁷ CFU/ml. The PBS may be divided into several tubs. The test piece may be put into the PBS, respectively. Samples of 2 ml of PBS are sampled respectively for culturing durations of 1, 4, and 8 hours. The samples are cultivated for 24 hours at a temperature of 37±1.0° C. and a relative humidity (RH) of not less than 90%. The number of living bacteria are counted by an agar culture method (37±1.0° C., 24 hours). FIG. 8 illustrates the histogram of the antibacterial analysis in number of living bacteria sampling according an embodiment of the present invention. Accordingly, the antibacterial property is proven again by the histogram. Furthermore, comparison of the change of the numbers of Staphylococcus aurous shown in FIG. 8 indicates that more titanium results in the quicker destruction of bacteria.

According to the results of corrosion resistance analysis and antibacterial analysis, evidence of corrosion resistance and antibacterial properties is provided. Thereof, stainless steel with cost competitiveness having superior antibacterial and corrosion resistance properties is provided by present invention.

As is understood by a person skilled in the art, the foregoing preferred embodiments of the present invention are illustrative rather than limiting of the present invention. Various modifications and similar arrangements are intended to be included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structure. 

1. A stainless steel having antibacterial and corrosion resistance properties, comprising: 12 to 30% by weight chromium to provide corrosion resistance; 0.1 to 1% by weight titanium; and 0.3 to 3% by weight copper.
 2. The stainless steel having antibacterial and corrosion resistance properties according to claim 1, wherein the stainless steel is a sheet, a strip, a pipe, or a wire.
 3. The stainless steel having antibacterial and corrosion resistance properties according to claim 1 comprising a base steel selected from the group consisting of austenitic, ferritic and martensitic stainless steel, wherein 12-25% by weight of Cr when said steel is austenitic, 12-30% by weight of Cr when said steel is ferritic and 12-18% by weight of Cr when said steel is martensitic.
 4. A method for manufacturing a stainless steel raw material having antibacterial and corrosion resistance properties, comprising the steps of: adding about 12-30% by weight chromium, about 0.3-1% by weight titanium, and about 0.3-3% by weight copper to molten stainless steel; performing a vacuum arc refining process; performing hot rolling and cold rolling; performing annealing and pickling processes; and performing a heat treatment.
 5. The method for manufacturing a stainless steel having antibacterial and corrosion resistance properties according to claim 4, wherein the hot rolling temperature is about 1000 to 1100° C. or less, and before hot rolling the hot rolling temperature is maintained for at least about 50 minutes.
 6. The method for manufacturing a stainless steel having antibacterial and corrosion resistance properties according to claim 5, the rolling reduction ranging from 50% to 60%.
 7. The method for manufacturing a stainless steel having antibacterial and corrosion resistance properties according to claim 6, wherein the resulting hot rolled steel strip is subjected to annealing before being cooled to room temperature prior to cold rolling.
 8. The method for manufacturing a stainless steel having antibacterial and corrosion resistance properties according to claim 4, wherein the annealing temperature is about 800 to 900° C. or less, and the annealing temperature is maintained for at least about 8 hours before cooling to room temperature.
 9. The method for manufacturing a stainless steel having antibacterial and corrosion resistance properties according to claim 8, wherein the resulting annealed steel strip is subjected to a pickling step in an about 55 wt % pickling agent.
 10. The method for manufacturing a stainless steel having antibacterial and corrosion resistance properties according to claim 9, wherein the pickling agent comprises HF and HNO₃ in a ratio of about 1:4.
 11. The method for manufacturing a stainless steel having antibacterial and corrosion resistance properties according to claim 10, wherein cold rolling is performed with a reduction of about 50%-60% after annealing and pickling the steel strip.
 12. The method for manufacturing a stainless steel having antibacterial and corrosion resistance properties according to claim 4, further comprising steps of heat treatment for about 30-120 minutes.
 13. A method for manufacturing a stainless steel having antibacterial and corrosion resistance properties according to claim 12, wherein a temperature of the heat treatment is about 600 to 800° C. 